Heat pipes are heat transfer devices with high effective thermal conductivities that are used to transfer heat from a high temperature region to a low temperature region by way of a heat transfer fluid (referred to herein as a “working fluid”), whereby a temperature at the high region may be stabilized or reduced.
Microchannel heat pipes are heat pipes having a closed channel with a small (often on the order of tens of micrometers) and angled (often triangular) cross section, and are partially filled with a working fluid (often methanol, ethanol, water, acetone, or ammonia). One end of the heat pipe (the “evaporator” section) is placed in contact with a relatively high temperature region of a host device (e.g., an integrated circuit or a system including both an integrated circuit and an adjacent heat sink), and the other end (the “condenser” section) is placed in contact with a relatively low temperature region of the host device. In operation, heat generated in the high temperature region of the host device is absorbed at the high temperature end of the microchannel heat pipe, causing liquid working fluid to boil. The relatively high pressure thus generated at the high temperature end forces the resulting vaporized working fluid towards the low temperature end of the heat pipe, where the vapor condenses again to liquid working fluid, thus releasing heat. The resulting difference between the curvature of the liquid-vapor interface at the hot and cold ends of the microchannel heat pipe results in a capillary force by which the liquid working fluid flows from the low temperature end back to the high temperature end.
Microchannel heat pipes are distinguished from conventional heat pipes in that conventional heat pipes must include a wicking structure to affect the capillary pressure difference, while in microchannel heat pipes the capillary pressure difference is a result of the small lateral dimensions of the elongated channel (central channel). The amount of fluid, cross-section size and shape, fluid properties, hot and cold temperatures, etc., determine the amount of heat that is moved from the high temperature end to the low temperature end. Heat fluxes of 10,000 W/cm2 have been demonstrated. Wire bonded micro heat pipe arrays have been fabricated with thermal conductivities up to 3000 W/mK, and models have shown that transient specific thermal conductivities of up to 200 times that of copper, and steady state thermal conductivities up to 2500 times that of copper should be possible.
Integrated circuits (ICs) are an example of devices that have been shown to benefit from microchannel heat pipes. As the feature size of integrated circuits (ICs) decreases and transistor density increases, the heat flux of ICs increases and thermal management becomes more difficult. This is true of conventional and high power electronic chips. Circuit performance degrades significantly as temperature increases, so effective thermal management is important. The addition of microchannel heat pipes to ICs has been shown in laboratory settings to provide effective thermal management.
Although the beneficial heat transfer performance of microchannel heat pipes has been demonstrated in laboratory environments, they are nonetheless not commonly used in commercial devices due to the high cost of incorporating the addition of microchannel heat pipes using conventional methods. One conventional microchannel heat pipe manufacturing technique includes etching or machining a channel in the device's (e.g., silicon) substrate, and then sealing the channel with a second wafer. Another conventional microchannel heat pipe manufacturing technique includes sintering to generate an array of parallel wires between metal sheets. Such conventional methods require significant changes to a conventional production IC fabrication flow, and therefore greatly increase the overall manufacturing costs of the resulting IC devices.
What is needed is a cost-effective method for producing microchannel heat pipes that can be efficiently incorporated, for example, onto an IC (e.g., as part of the IC fabrication process, or produced on the IC after the IC fabrication process, or fabricated on a separated substrate that is then attached to a fabricated IC). What is also needed are inexpensive microchannel heat pipes formed by the method, and devices that are modified to include such inexpensive microchannel heat pipes.